1. Field of the Invention
This invention relates to a displacement sensor for detecting displacement of a measured magnetic object and, particularly, to an inductance-type displacement sensor which is little affected by noise resulting from an external magnetic field.
2. Prior Art
An inductance-type displacement sensor can be used for detecting displacement of a measured object made of a magnetic material. FIG. 1 shows an example of such a displacement sensor, FIG. 1(a) being a sectional view thereof positioned adjacent to a measured object and FIG. 1(b) being a side view. As shown in these figures, the displacement sensor includes a sensor core 100 having an E-like shape in section and integrally formed with a post-like magnetic pole 101, a cylindrical magnetic pole 102 positioned to surround the magnetic pole 101 and a base portion 103 connecting both magnetic poles, and a sensor coil 104 wound around the central magnetic pole 101. A dummy coil 105 provided outside the sensor is connected in series with the sensor coil 104, as shown in FIG. 2, and a carrier wave from a carrier wave generating circuit 106 is applied to the series circuit of the sensor and dummy coils 104 and 105. A detection circuit 107 is connected parallel to the dummy coil 105.
When a measured object of magnetic material 110 is displaced towards or away from the displacement sensor, the inductance of the sensor coil 104 varies. A change in electric potential across the dummy coil 105 due to a change in inductance of the sensor coil is detected by the detection circuit 107 whereby the displacement of the object 110 is detected.
Using such an inductance-type displacement sensor, a displacement detecting system can be made for detecting a displacement of, for example, a cylindrical or post-like shaft made of a magnetic material. FIG. 3 shows four sensor cores 211, 212, 213 and 214 which are arranged on two orthogonally intersecting axes X and Y and spaced circumferentially about measured magnetic shaft 220. Sensor coils 201, 202, 203 and 204 are wound on the sensor cores 211, 212, 213 and 214, respectively.
When the shaft 220 is displaced from a predetermined position in a Y-direction, for example, the inductance of the sensor coils 201 and 203 is varied, and when displaced in an X-direction, then the inductance of the sensor coils 202 and 204 is varied. By detecting the changes in inductance, the displacement of the object 220 is detected. For this purpose, a pair of opposite sensor coils is connected in series to a carrier wave generating circuit, and a detection circuit is connected in parallel to either one of the sensor coils connected in series. FIG. 4 shows an example of such an arrangement in which the sensor coils 201 and 203 are connected in series with a carrier wave generating circuit 215 and thus a magnetic flux 206 is generated by the sensor coil 201, for example. A detection circuit 216 is connected in parallel to the other sensor coil 203. If the shaft 220 is displaced in the Y-direction, the inductance of the sensor coils 201 and 203 is varied and this variation in inductance is detected by the detection circuit 216.
When the inductance-type displacement sensor shown in FIG. 1 is operated in a location in which an external magnetic flux EFX passes through the sensor core 100, as shown in FIG. 5, the external magnetic flux EFX causes an electromotive force EMF to be generated in the sensor coil 104 by mutual induction, as shown in FIG. 2. Since this electromotive force EMF is superimposed on the change in electric potential resulting from the change in inductance due to the displacement of the measured object 110, a noise is superimposed on the displacement signal to be output from the detection circuit 107.
Such a situation also takes place in the displacement detecting system shown in FIG. 3. Since a pair of opposite sensor coils is connected in series, as shown in FIG. 4, an electromotive force EMF is generated, as shown in FIG. 4, in the sensor coil 201 by the mutual induction when an external magnetic flux 207 is received by the sensor coil 201. The signal generated by this electromotive force is superimposed on the change in electric potential resulting from the change in inductance of the sensor coils 201 and 203 due to the change in position of the shaft 220, and thus a noise is superimposed on the displacement signal output from the displacement sensor.